This is a special. -- from the ABC. -- -- Cutler New York with a CBC news digital special report Russia tightening its grip its grip. And Ukraine's Crimean peninsula today as the potential for armed conflict in that region. Is growing by the hour British foreign secretary William -- is -- situation Ukraine the most dangerous crisis in Europe this century. Meantime the Russian foreign minister Sergey Lavrov says Russia intervened. Because Ukraine because of the live the lives of its citizens there were under threat. BBC's Christian preacher has more from Sevastopol. On the -- blacks. The pride of the Ukrainian navy on the -- ripped from the rule of the stuff headquarters today -- -- Russian crowd. The response to losing patience -- The -- surrounded by Russian soldiers. And yesterday this -- admiral Dennis that is husky head of the Ukrainian navy switch sides squaring his allegiance to Crimea and -- day. Unity and admirable and well today he returned and urging others to follow it. But he underestimates the result of his success says standing next to him and the men and women he wants -- it. NN together in defiance they song Ukrainian companies. Ukraine has not yet died at the words hello to a hospital -- But outside the crowd grew ever more restless they chanted insults that the young Ukrainian soldiers inside. Told me they was staying post. Ukraine Ukrainian fuel for Ukraine together and ethical business but anyway and you -- yes. -- -- -- -- How often -- the Ukrainian rank and file isolated on the siege their loyalties tested and -- -- There they go with those offices who sworn allegiance to the -- prior Russian. Authority here in Crimea -- -- they resist as many seem willing to -- in the face of insurmountable odds he didn't. Amid the confrontation. A brotherly -- behind from a Russian soldier. But there's no sign Moscow is relenting -- -- on the Ukrainian coastal defense division is still pending. And a cross the peninsula troop buildup continues its. -- -- -- cash say they can see military vehicles massing on the Russian side of the border. Crimea is lost Kiev knows it but how far the president Putin's ambitions stretch. With Crimea one might he turned to east in Ukraine. And is there anything to stop it. Christian praise -- BBC news Sevastopol. Several isn't what's happening in Crimea wanna go to two ABC's outsmart -- was on the phone joining us now. From Crimea Alex pedestrian with his efforts -- what is the situation right now -- how much control does Russia -- Well. UB BC some bright buried in it is law for all intents and purposes Russia would essentially. Taking control. Over the top seeded of this entire peninsula it started a few days ago. When it took control of the parliament they are great forces brother took control of the airport -- -- -- the -- the controlling the border crossing. -- -- minute search here who. This the especially sworn allegiance. And so they really do control they've -- majority of territory to. Russian citizens. Here on the ground tours who organized -- source at a roadblock on the main road. They essentially control all but if you military bases here Crimea. -- essentially we've also been hearing reports that Russia has giving. Ukrainian forces in Crimea a deadline -- ultimatum if you will at 10 o'clock eastern time 5 o'clock tomorrow morning local time. To stand down to give up their bases to Russian forces. How strong how significant other of a deadline is that. Well they're still a lot of confusion surrounding that deadline it just came out a little a little while ago and it would be. The resources from the Ukrainian defense ministry that -- had not yet been confirmed by the Russian defense Ministry's fact. That Russians told. A leading newspaper in Moscow -- it is. Utter nonsense we immediately reached out to a base commander who met yesterday Ukrainian. Who are not yet surrender is basically says that she had heard nothing about that ultimatum and regardless -- -- ticket orders from the Russians. And it did its part -- don't ukrainians certainly not given. Orders -- remaining bases -- surrender. We know that there is been able headquarters in the oldest still being held by Ukrainian along with five other military base this. On the peninsula. That the -- that are on the ground now who are they are they standard infantry men. And it's -- -- a paramilitary force. It's still unclear how -- -- didn't days later it's really quite remarkable. -- -- highly professional well organized heavily armed force. That has taken control. This infrastructure. We believe -- certainly would be Russian forces if not. In the actual Russian army -- -- Russian contractors. They're very much on the side -- Russia that much is clear. What about is far as the Ukrainian forces if in Sevastopol or some -- a -- would they said any chance. If in fact a conflict breaks out. -- if push comes to show up you're talking about whether. A small number are relatively small number let's say a Ukrainian forces can hold her own against -- got resources of the Russian army. The Russian did it in shorter Russia would it would it would comment and then take them easily. Russia party has some 151000 troops here the -- the black but usually we see those numbers increase. Over the -- few days. They are they are not -- they're certainly -- gone -- Russia of course being extremely close if they wanted to. -- it and take a Crimean peninsula once and for all it wouldn't take all that -- There's been a lot of discussion here the United States about sanctions against Russia canceling the coming G-8 meetings and kicking Russians out of the -- altogether. Any chance that something like that would have any kind of change in what's going on from your perspective. That's the million dollar question an -- and the question there is whether gluten is bothered by. What it essentially is really bad back -- he has spent years and millions of dollars. I'm to improve Russia's image in the world that it and a cherry on top if you will what this Sochi games. And in so and that went very well now -- -- -- -- all the way so. -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- Certainly did not foresee any sort of military action you'll see American boots on the ground. So what they get there and do they talk about economic sanctions they talk about pulling out of or in the -- the -- to talk about pulling out of the EA corporate and Sochi later this year and the questions. That becomes does prudent bear with -- -- -- to that sort of the really the ball is really entirely in Putin's court he can. He didn't pull back all the district who are you want to or give mark the full scale war and they're not being that -- the world knew about it. We'll have to wait and see that in fact how that is going to play out while on the diplomatic front and they're right in Crimea where were ABC's -- -- -- station -- thanks for that. But what about the states -- the Pentagon where ABC's digital reportedly Martinez spent standing by and monitor the situation there and -- Back to those reports of that military ultimatum that was put forth by Russia delivered by Moscow to the crane forces in Crimea -- US officials. And they said they got anything to suggest that that this is true that they're preparing for so. -- storm and action. The and it really don't have any good information about this as Alex indicated on his phone -- -- this is a lot of circular reporting. Obviously is that coming from Ukrainian sources then being reported by Russian sources so it's very unclear officials here depending on whether there's any validity to this. On the net and there you have to take any development -- seriously and that's been happening since last Thursday when we first got reports that. They were inbound flights into. Airports at tech in the -- with troops abort it took a couple of hours but US officials -- -- able and determined that yes there was information about that that was accurate. -- weren't in a position to -- provide an assessment. Those. Inflows continued throughout the weekend unabated. And that's why we -- finally ended information over the weekend that the Russians are probably put in and an additional 6000 forces. But -- this latest ultimatum that there is really no confirmation from this side. That it is accurate Russia's grip already strong and gotten even stronger you mention those additional troops admit their way into the -- -- -- well. Basically -- all the pressure points in -- Crimea the airports to seaports major roadways. Is it clear that what it will take to get my hospital loosen that grip to pull out their troops. Well militarily they seem to be in control officials saying yesterday that Russia had complete operational control within the Crimea. What of course says Alex -- indicated they were still some hold out. Opposition's bases of Ukrainian military forces that are based in Crimea that are holding out there have been no reports of violence which is a good thing. Which it because that is what the United States has been urging on all fronts every single American official. It was -- in contact with -- senior Russian government officials has been urging that this be resolved peacefully. The latest development here is a Vice President Biden called. The prime minister muted and -- and urged him to pull out Russian forces and to talk with this transitional government in Kia and just resolve this matter peacefully. But in terms of what's actually happening on the ground there is it it's obvious that Russia is Russian forces. Are in charge though again is Alex said these guys are not wearing. Insignia on their uniforms it's unclear exactly precisely who they are secretary of state John Kerry who is headed for the capital Kiev tomorrow he had this to say. Sunday on ABC's this week. Russia has engaged in and military act of aggression against another country. And it has huge risks it's a nineteenth century act in the 21 century the president is currently considering. All options they're all on the table. -- they key phrase there that a lot of world leaders and frankly a lot of citizens whenever they hear those world leaders say that all options on the table is that accurate. From what you've heard it what are the more likely options -- -- of retaliation against Moscow. Well as a -- -- -- indicates that the United States is willing to consider any option but of course what we're hearing from senior officials is that. The military option is not being perceived here in any form or way. This is a diplomatic track that is going to be pursued against Russia urged them to pull out their forces and to talk with -- Ukrainian leadership. That's basically what's happening here -- that there's been no planning as far as we can tell. About potential military movements American military movements in response to this. And it's it's pretty and it's pretty simple reason why you don't want to escalate something when it doesn't have to. Obviously this is already very serious situation you don't wanna make it worse Lewis speaking at a US and involvement or -- the potential is the presence anywhere where the nearest American troops. The weapons installments where they had in the region. Well the United States has a very good presence with good size -- in Western Europe. Government remember Europe. After what -- to the United States at a very large presence that's been downsized over the years so the main areas where the United States has forces. Are in Italy Germany and in England. In terms of the air assets in England a lot of air and ground forces in Germany. And the Italy -- -- air force bases as well is an army base -- 173 airborne that can respond quickly. -- there are also naval assets in the region well remember those two ships were sent into the black single one of them still in the Black Sea. Unfortunately it's out of commission because this is that ship that ran aground. But there are other smaller ships that in the -- and we just had coincidentally now. An inflow the USS. George W. Bush -- America's large carriers -- -- inbound to the Middle East in order to get to the Middle East it has a -- after the mad. Who's gonna spend a couple weeks there anyway. But they're just proceeding under normal operations right now. No. Opinion right now you're depending on the they're going to be moved. Any closer to what's happening Black -- region and missile bases in Poland and Hungary as well correct. That's right that -- are there are with ground based interceptors in Poland. Very small numbers of American forces there these this is part of that what they call the phase defense a missile defense against the -- potential of any long range Iranian missiles. So those missiles are directed. As a protective measure -- Eastern Europe. And -- in this has been a bone of contention with the Russians for years now they have seen those of the presence of those missiles as a threat. The United States has been adamant that these missiles are there to -- and Eastern Europe against the missile threat from Iran. And I don't CNET that it could be used potentially -- -- all right ABC's Martinez at the Pentagon -- thank -- insight appreciate that. For more on the crisis in Crimea and the latest on Russia's threat to Ukraine and what the west could potentially do about -- stay right here -- abcnews.com. This is an -- special report. I'm Dan Cutler New York.

This transcript has been automatically generated and may not be 100% accurate.

Russia's Involvement in Ukraine Worries Western Leaders

Claims surface that Russian forces have secured an airport in the Crimea region of Ukraine.

Russian Forces Officially Enter the Crimea Region of Ukraine

President Obama calls Vladimir Putin as tensions in Ukraine intensify.

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